In order for me to figure reasonable dose, I'll need to get some kind of estimate of the gallons your tub holds. You can determine that by:

1. Adjusting the flow to a reasonable level, with the drain open.
2. Filling an empty milk jug and timing how long it takes to fill.
3. Closing the drain and then filling the tub, and timing that interval.
4. Then, dividing the tub interval by the jug interval to give approximate gallons.

Be sure to have her get in, and make sure you haven't selected an overfilled level.

With many tub surfaces, you can mark the line with a magic marker, and then remove the mark with rubbing alcohol. But, some surfaces are porous, so test this with a small inconspicuous mark, before you do anything more visible.